How much fiberglass over plywood?

Discussion in 'Boatbuilding' started by San Juan Sailor, Mar 30, 2009.

  1. San Juan Sailor
    Joined: Mar 2009
    Posts: 6
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: San Juan Island, WA.

    San Juan Sailor Junior Member

    I'm replacing the plywood decks, cabin top and sides on the boat I'm restoring. How many layers and what thickness of glass to use? I thought I'd use System Three but read in this forum about the US Composites epoxy being alot cheaper. Any opinions?
    Dan
     
  2. PAR
    Joined: Nov 2003
    Posts: 18,409
    Likes: 366, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    All of the major formulators of epoxy offer products with similar physical properties so use the goo you're comfortable with.

    On a deck that doesn't need the additional support of 'glass and is strong enough without it, the fabric serves as abrasion protection. In this case you don't need a lot. Typically 8 ounce fabric is fine, though a little thin for high traffic areas or workboats. Two layers of 6 or 8 ounce can increase the abrasion protection, though costs of fabric and resin go up.

    Other fabrics, such as Dynel and Xynole are commonly used on decks. Both drape over compound curves much better then regular fabrics and both are much higher in abrasion resistance. Unfortunately they also require a lot more resin to wet out, as much as three times more then similar weight conventional fabrics.

    In the end, you'll have to decide how much protection an area needs and how much effort or abrasion resistance you can afford. Dynel is about 20% more then regular fabrics, Xynole is considerably more and both will suck up resin like a ***** outside a GM plant on payday.
     
  3. rasorinc
    Joined: Nov 2007
    Posts: 1,849
    Likes: 69, Points: 48, Legacy Rep: 896
    Location: OREGON

    rasorinc Senior Member

  4. San Juan Sailor
    Joined: Mar 2009
    Posts: 6
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: San Juan Island, WA.

    San Juan Sailor Junior Member

    Glassed plywood decks

    I was told at Fiberglass Supply where I picked up some System Three that one layer of 5.6oz would work. I'm not worried about adding strength so much as weather proofing but your point about wear resistance is a good one. I want to do some kind of non skid and that would help with the wear issue. Any good ideas for non skid on a glass and epoxy deck?

    Here is a link to a blog I started on this boat project;
    http://www.sailblogs.com/member/rebuildingmariner32/
     
  5. PAR
    Joined: Nov 2003
    Posts: 18,409
    Likes: 366, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    6 ounce will work, but it's light so abrasion resistance will be limited.

    If you're attempting to put a 'glass sheathing over a laid deck on that old ketch, then you need to do one of two things. Replace ether decking substrate with plywood, then apply the 'glass sheathing or apply a much heavier fabric laminate.

    The reason is simple, a laid deck (a bunch of individually laid boards, fastened to the deck beams) has a lot of movement. This deck planking movement will rip loose from the sheathing creating a lovely place for rot to live, fester and raise it's ugly head in a few years. So, you have to lock it down with a much thicker sheathing and solid fastening to the deck beams (the old planking).

    In other words if the deck is marginal, remove it and replace with plywood, then apply a 6 to 8 ounce sheathing. If the old deck is in fairly good shape, insure the fasteners are tight and apply a thick sheathing (and hope for the best). How thick, well 16 ounces at least, better yet would be Dynel or Xynole, which have a much higher modulus of elongation and can absorb much of the movement before sheering loose.

    If I remember correctly Mariners were plywood decks with polyester set sheathings which as we now know doesn't work, though at the time the jury was still out on longevity. Epoxy should cure this issue.
     
  6. San Juan Sailor
    Joined: Mar 2009
    Posts: 6
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: San Juan Island, WA.

    San Juan Sailor Junior Member

    The deck is not marginal, it's totally rotten and delaminated and I'm replacing all the original plywood. I guess in 1972 they didn't know about water proof glue and yes the original glass was put down with polyester.
     

  7. PAR
    Joined: Nov 2003
    Posts: 18,409
    Likes: 366, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    The plywood was WBP glue (most likely), but even the best plywood will rot if subjected to moisture long enough.

    Use good marine plywood, fully encapsulated in epoxy, with a minimum of two coats (much better if three) on every surface, especially the edges.

    The fastener holes in the deck beams will also need to be drilled out, a dowel fitted (epoxied) and sanded flush. The majority of these holes will be shot (rot).

    Back in the era your boat was built, there was a run of plywood (from the far eastern countries of course) that used a fish oil based adhesive. It didn't hold up and a lot of boats had delamination issues, including major yacht manufactures. I owned one of these boats once and had to replace every bulkhead, cabinet, counter top, furniture, etc. Fortunately the hull and deck where 'glass. If your boat was built in the far east (it was, again if memory serves me), then all plywood should be considered "suspect" and replaced.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.